It has long been recognized that locking clutches employing sliding clutch rings ideally should provide quick and positive engagement between the vehicle's driving member and the vehicle's driven member. A clutch providing quick and positive engagement between these members minimizes the problems resulting from a "snap start" condition. This condition typically occurs when the vehicle makes a transition from the two wheel drive mode to a four wheel drive mode with the sliding clutch ring being partially engaged to both the driving and driven members. As this occurs, in certain instances the vehicle's power source attempts to transmit more torque to the clutch ring than the clutch ring can withstand when in the partially engaged condition. As a result, the clutch fails to lock up with the result being that the clutch ring becomes damaged and rendered inoperable.
Prior attempts to produce locking clutches with sliding clutch rings have utilized either a moving cam type arrangement or a fixed cam type arrangement to activate the sliding clutch ring and produce clutch lockup. In general, a moving cam type arrangement is inherently slow to lock the driving and driven members. The principal reason is that a moving cam cannot be resisted with sufficient torque to rapidly effect clutch lockup without generating excessive heat while running in the locked condition. The heat must be dissipated by the vehicle's axle, with the effect being that the heat can burn out the vehicle axle bearings. Consequently, a locking clutch with a sliding clutch ring activated simply by a moving cam type arrangement does not produce a quick and positive lockup and thus is susceptible to the "snap start" problem. Typical prior art moving cam type arrangements are shown by Kagata, U.S. Pat. No. 4,163,486, and Goble, U.S. Pat. No. 3,656,598.
Locking clutches employing solely a movable cam type arrangement are susceptible to yet another problem. In the two-wheel drive mode, especially in cold weather situations where lubricants between the driving and driven members are cold, the driving member tends to rotate with respect to the driven member. In some instances, the rotational movement of the driving member may be as much as one-half the rotational movement of the driven member. Locking clutches employing the moving cam type arrangement in such a situation tend to effect clutch lockup. This becomes a major problem especially when the vehicle is moving at significant speeds.
Fixed cam type arrangements overcome these problems by typically producing faster lockup than moving cam type arrangements while resisting the tendency to inadvertently lock up when the vehicle is operated in the two wheel drive mode. Once the driving and driven members are in the locked condition, one came surface moves with respect to the other resulting in a chatter or a "ticking" between the cam surfaces. This typically results in a very high wear with the driving and driven members jumping in and out of a locked condition. Locking clutches utilizing solely fixed cam type arrangements are shown by Petrak, U.S. Pat. No. 3,217,847.
The present invention solves these problems of the prior art by providing a locking clutch utilizing a sliding clutch ring activated by both a fixed and moving cam. The sliding clutch ring initially is urged into an engaged condition between a driving and driven member by a fixed cam. Subsequently, once the locking clutch has effected engagement, a moving cam increases the engagement and maintains the locked condition. In this manner, the present invention provides a locking clutch having the quick and positive locking characteristics provided by a fixed cam as well as the advantages provided by a moving cam in maintaining a locked condition.